Conductive nonwoven fabrics are used as gas diffusion layers in membrane electrode assemblies (MEAs) for PEM (proton exchange membrane) fuel cells and should be electron conductive and gas-permeable. These nonwoven fabrics should also have channel structures for transporting away water that is formed during a reaction when operating fuel cells, and should possess an inherent rigidity at a limited thickness. Furthermore, these nonwoven fabrics should have a good surface smoothness at least on one side.
Two-dimensional textile materials, such as conductive woven fabrics or wet-laid nonwoven fabrics, may be used as starting materials for gas diffusion layers. Nonwoven fabrics having a thickness of 0.3 to 0.5 mm are referred to in Japanese Patent Application No. 06/123050, these fabrics including carbonizable polymer fiber materials, such as polyacrylic nitrile (PAN) or oxidized polyacrylic nitrites, with a mass per unit area of 100 to 200 g/m2. To achieve the required electrical conductivity, these textile fabrics are carbonized at temperatures between 1000 and 2100° C., whereby a carbon content of approximately 90 to 96% is achieved. A reduction in mass per unit area by 30 to 60 weight-percent is connected with the carbonization, i.e. graphitization process. Usually, these fabrics, which are now conductive, are rigid and demonstrate a relatively open structure. The woven fabrics or nonwoven fabrics may be impregnated or coated with dispersions of conductive particles, such as graphite or carbon black, to achieve the required rigidity and to improve the conductivity in the X, Y, and Z direction, and subsequently to press them to achieve a high degree of surface smoothness.
Furthermore, Japanese Patent Application No. 10/777624 and Japanese Patent Application No. 10/777625 refer to the production of gas diffusion layers directly from carbon fibers in a wet-laying process, so that subsequent carbonization of the precursor fibers is unnecessary. In this connection, polyvinyl alcohol (PVA) solutions or polyethylene terephthalate (PET) substrates may be used as a binder for the carbon fibers. Subsequently, the hydrophobic properties of the gas diffusion layer may still be adjusted by finishing them with a hydrophobization agent, such as polytetrafluoroethylene (PTFE) dispersions and subsequent sintering.
It is believed that methods for the production of gas diffusion layers for PEM fuel cells are disadvantageous in that they may not be rolled up for transport and processing and must be filled with conductive fillers, to achieve the required conductivity values.